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Muscle Fibers 101

Knowing the types of fibers that make up your muscles can help you dictate how to best train for specific goals, activities, or sports. Here is the lowdown on your muscle fibers …

Type I

Slow-twitch muscle fibers work hard over longer time durations and don’t contain as much power as fast-twitch muscle fibers. This means that your Type Is are utilized more during endurance-focused activities such as running, cycling, and swimming.

Train: More reps and not a ton of resistance. Think circuits, bodyweight moves (planks, pushups, air squats), or training that relies on imbalances, such as one-leg squats, Romanian deadlifts, or box jumps.

Type IIX

Your fast-twitch muscle fibers are utilized during explosive movements like sprints, vertical jumps, and quick cuts. These also tire quickly compared to Type I fibers.

Train: Go heavier with weight, lower in reps. Use a Tabata protocol (20 seconds on, 10 seconds off for 4 minutes) or explosive moves like kettlebell swings or KB cleans.

Type IIA

Also known as fast twitch oxidative glycolytic, your Type IIa muscle fibers are a combo of Type I and Type IIx fibers. So, they last longer than standard Type I, but tire quicker than Type IIx. They contract fast, have an intermediate resistance to fatigue and are generally used for long-term anaerobic activity.

Train: In terms of training, most of your muscle groups fall into the combos region. While slow-twitch are first to kick in when the muscle contracts, if they can’t get the job done then Type IIs will take over to help out. Mix it up or keep your training in the middle — semi-heavy weight with a mid-grade rep range (say, lower than 15 but higher than 6). Aim to spend about 1-3 minutes recovering.